Unshielded twisted-pair (UTP) cables and shielded twisted-pair (STP) cables are commonly used in local area network (LAN). Generally, UTP cables are more susceptible to crosstalk than STP cables. Crosstalk occurs in twisted pair cabling when energy from one wire pair radiates and couples itself on another adjacent wire pair. In a simplified model, each of the non-transmitting wire pairs acts like an antenna with at least one of the pairs transmitting.
Crosstalk is caused by several factors such as proximity of wire pairs, wire quality, and the number of twist per distance. Standards for UTP and STP cables relating to wire quality and wire application have been developed by the Electronic Industries Association and the Telecommunications Industry Association (EIA/TIA). Among the most popular is the EIA/TIA-568 Standard, which classified twisted pair cables into 7 different categories (Category 1-7).
There are two types of crosstalk: near-end crosstalk (NEXT) and far-end crosstalk (FEXT). In NEXT, crosstalk is measured at the end where one pair is transmitting. In FEXT, crosstalk is measured at the opposite (far) end. FEXT measures the effect of crosstalk from one pair to another over the length of the cable. Two common methods used to measure NEXT crosstalk are pair-to-pair and power sum. In pair-to-pair, the maximum interference caused by one of an active cable pair is measured. In a four pair cable, this method is implemented by having only one of the pairs transmitting. In the power sum method, crosstalk is measured on each pair while all other pairs are transmitting. This latter method is the more preferred method in measuring crosstalk because, in a typical network, data are transmitted over multiple pairs simultaneously.
Several methods are used to minimize crosstalk such as varying the twist rate in each of the wire pairs, terminating the twist as close to the cable terminals at possible, and utilizing adaptive digital crosstalk cancellers.
Adaptive digital crosstalk filters (cancellers) automatically adjust the coefficients of a digital filter to match the coupling function between two channels (two wire pairs). A transceiver has approximately 24 filters one for each possible combination of couplings across a four wire pair cable. Typically, all 24 crosstalk filters are turned on. However, in situations where STP cables are used, all 24 crosstalk filters are not needed. Accordingly, what is need is a transceiver that could determine the cable type being used and to turn the filters off to save power.